Further investigation on this sample was performed in a Nion UltraSTEM
100 operated at 60 kV. Fig. REF a shows an atomically resolved
medium angle annular dark field (MAADF) image of a small portion of
the suspended heterostructure. In the range of scattering angles used
here (ca. 60 to 200 mrad), regions where the atoms are precisely on
top of each other appear brighter because the intensity here is not
simply the sum of the two overlaid lattices, as it would be in a high
angle annular dark field (HAADF) image. [1]} Indeed,
in HAADF imaging (ca. 80 to 240 mrad) the intensity does not vary
across the differently stacked regions (see supplementary Fig. 1).
For this reason, and also because the MAADF image has a better signal-to-noise
ratio than the HAADF image, MAADF imaging was preferred over HAADF
in this work. There are three types of high-symmetry stacked regions
labeled as AA (C atoms aligned with B and N atoms), AB (C atoms aligned
with B atoms only) and AB' (C atoms aligned with N atoms only). The
top view structure models of the three stacking types are schematically
shown in Fig. REF b-d. In Fig. REF f-h the AA, AB
and AB' regions are shown at higher magnification in red, cyan and
green frames, respectively. A STEM simulation of the heterostructure,
performed using the QSTEM software,[2]} is shown in Fig. REF e
and the AA, AB and AB' regions are shown at higher magnification in
Fig. REF i-k. Fig. REF l-n show the intensity
profiles for each of the three regions along the yellow lines of Fig. REF f-k
for the experimental (solid line) and for the simulated (dashed line)
case. The AA region can be identified already from its visual appearance,
which is distinctly different from that of the AB and AB' regions
(compare Fig. REF f,i with Fig. REF  g,h,j,k).
The AB and AB' region can be distinguished by comparing the intensity
modulation in the lattice, which is always stronger in the AB' region
(where C and N are aligned, Fig. REF n) than in the AB region
(where C and B are aligned, Fig. REF m). Hence, from the
appearance and relative intensity variations (Fig. REF l-n
are plotted with the same intensity scale), it is possible to unambiguously
associate each moirÃ© spot to a specific stacking type. After careful
analysis of many regions across the sample, we observed that the AB
stacked regions consistently appear larger than the AA and AB' regions.
This can be clearly seen for instance in Fig. REF a,
where a MAADF image containing several moirÃ© spots is presented (a
black mask was used here to cover contaminated areas). The three different
moirÃ© regions were identified as explained above for Fig. REF .
Individual AA, AB and AB’ regions are enclosed by
red, cyan and green polygons respectively. The sides of the polygons
are placed approximately along the lines of minimum intensity between
two adjacent moirÃ© regions. Already at a first glance, it is evident
that the AB region is the largest of the three. Indeed, as drawn in
Fig. REF a, the AB region measures 55 nm2,
while the AA region is 34 nm2, and the AB' region
is 32 nm2.
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